Poly(arylene sulfide) (hereinafter sometimes abbreviated as “PAS”) resins including polyphenylene sulfide (hereinafter sometimes abbreviated as “PPS”) resin are resins superior in functional balance of heat resistance, chemical resistance, flame retardancy, mechanical strength, electrical properties such as insulation property, dimensional stability, and the like. The PAS resins have been commonly used as resin materials in a wide variety of fields such as electrical and electronic instruments, automotive instruments and chemical instruments because they can be molded or formed into various kinds of molded products, films, sheets, fibers, etc., by general melt processing processes such as injection molding, extrusion, and compression molding. Since the PAS resin can be used continuously especially under a high temperature environment of 170° C. to 190° C., one of the applications of the PAS resin is a bag filter of a dust collector in which fibers of the PAS resin are used. Since the temperature of the exhaust gas that comes into contact with a bag filter may become a high temperature as high as 140° C. to 250° C., the filter material of a bag filter is required to have heat resistance. Moreover, since exhaust gas may contain acidic gas or moisture, the filter material of a bag filter is also required to have acid resistance and resistance to hydrolysis. The PAS resin has been increasing in use as a high performance fiber suitable for this application. Moreover, it is necessary to prolong the life in use of a bag filter and maintain high capture efficiency by, for example, removing soot and dust attached to the bag filter by the use of mechanical vibration or back gas flow, it has been demanded to stably provide a PAS fiber superior in spinnability.
Heretofore, only PAS resins with a low molecular weight (having a melt viscosity of about 10 Pa·s or less) have been able to be produced by ordinary polymerization methods as PAS resins for forming PAS fibers, there has been used a method in which heat treatment is performed in the presence of oxygen, thereby achieving a melt viscosity needed as a crosslinked type PAS resin or a method in which molecular weight is increased by causing a crosslinking agent to exist during polymerization. Such PAS resins with increased molecular weight are inferior in extrusion processability due to their high degree of crosslinking or branching and it is difficult to form fibers or the like therefrom. A straight-chain PAS resin with high molecular weight has been developed by the use of a polymerization aid or a phase separation agent and PAS fibers made of straight-chain PAS resins are known. In such straight-chain PAS resins, mechanical properties such as heat resistance and strength and moldability such as spinnability are in an opposite relation. For example, Japanese Patent Application Laid-Open (JP-A) No. 63-315655 (Patent Literature 1) has disclosed a melt-blown non-woven fabric made of a straight-chain PPS having a weight average molecular weight of 20000 to 70000 in order to eliminate drawbacks of a conventional thermally-crosslinked PAS resin or a PAS resin obtained by performing polymerization in the presence of a crosslinking agent. If the weight average molecular weight exceeds 70000, then spinning conditions including a temperature considerably higher than the melting point are required, resulting in the occurrence of decomposition or gelation of a polymer and consequently lacking in spinning stability due to clogging of a nozzle. Conversely, if the weight average molecular weight is less than 20000, then the melt viscosity becomes extremely low only by using a spinning temperature slightly higher than the melting point, and therefore the range of controlling spinning conditions is narrow and also the strength of a non-woven fabric was low.
As a PAS resin, one having been three-dimensionally crosslinked by copolymerizing a polyhaloaromatic compound such as trichlorobenzene has been known. For example, Japanese Patent No. 2514832 specification (Patent Literature 2; equivalent to U.S. Pat. Nos. 5,200,500 and 5,268,451, and EP 0344977 A2) has disclosed a PAS crosslinked polymer in which a polyhaloaromatic compound has been made react. This crosslinked PAS resin can be used as a polymer modifier, such as a flashing inhibitor in injection molding, a weld strength improver, and a crystallinity improver in injection molding, by being added to a PAS resin composition, it has been reported to appear approximately gel-like in a molten state and therefore it has not been able to be melt-molded or spun alone. WO 2006/068159 A1 (Patent Literature 3; equivalent to EP 1837359 A1) has disclosed to obtain a branched PAS resin having a melt viscosity, measured at a temperature of 330° C. and a shear rate of 2 sec−1, of 10.0×104 to 40.0×104 Pa·s and a melt viscoelasticity tanδ, measured at a temperature of 310° C. and an angular velocity of 1 rad/sec, of 0.10 to 0.30 by adding a polyhaloaromatic compound and a phase separation agent to a polymerization reaction mixture at the time when the conversion of a dihaloaromatic compound has become 80% or more. This branched PAS resin can inhibit the occurrence of flashing remarkably and can afford a molded article superior in surface property when being blended as a polymer modifier into a thermoplastic resin such as a straight-chain poly(arylene sulfide) resin, but it is extremely high in melt viscosity and therefore it has not been able to be melt-molded or spun alone.
JP-A No. 2009-270230 (Patent Literature 4) has disclosed a method for producing a PAS fiber in which in performing the melt extrusion of a PAS resin, a measured value of the activation energy of the viscosity of a melt is fed back within an extruder to the rotation speed of the screw of the extruder, thereby adjusting the activation energy; however, this method requires an extremely complicated control and therefore has problems with operativity or productivity. JP-A No. 2009-270219 (Patent Literature 5) has disclosed a PAS fiber having a total value of peak areas, measured by HPLC (High Performance Liquid Chromatography) analysis, of less than 1.4×106, which is an index of the amount of low molecular weight matters contained in a polymer; however, a complicated process management has been required, for example, the provision of a vent in a melt-extrusion step.
As described above, the conventional straight-chain PAS resin, branched PAS resin, and crosslinked PAS resin are all difficult to be spun to form a fiber or be melt-molded into a film or the like alone.